Journeyman Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A service entrance conductor for a commercial building is installed underground in a rigid metal conduit (RMC). The service is 480Y/277V, 3-phase. What is the minimum cover depth required for this RMC, assuming it is not subject to vehicular traffic?
- A6 inches (150 mm)
- B24 inches (600 mm)
- C18 inches (450 mm)
- D12 inches (300 mm)
Show answer & explanationAnswer & explanation
Correct answer: A. 6 inches (150 mm)
According to NEC Table 300.5, Column 5, for RMC containing conductors of 0-1000V, the minimum cover depth is 6 inches when installed under a building, or 6 inches when installed in a trench under a public or private driveway (with specific conditions). However, for general burial not subject to vehicular traffic, Column 5 lists 6 inches for RMC. This is a common exception for RMC and IMC.
Why the other options are wrong
- B. 24 inches is for specific conditions or higher voltages, or direct burial cables under roads.
- C. 18 inches is a common depth for direct burial cables or other conduits in general locations.
- D. 12 inches might apply to other wiring methods like PVC or direct burial cables in specific conditions.
Underground Wiring Cover Depth (RMC)
The NEC specifies minimum cover depths for underground wiring methods to protect conductors from physical damage, with rigid metal conduit (RMC) often having reduced requirements due to its inherent strength.
- NEC Table 300.5 dictates cover depths.
- Depth varies by wiring method (RMC, PVC, direct burial).
- Depth varies by location (under buildings, under roads, general).
- RMC and IMC often have shallower depth requirements.
Memory trick: Conduit type, voltage, and traffic tell the burial depth story.